For a fuel cell system mounted on vehicles and the like, there have been proposed various hybrid fuel cell systems provided with a fuel cell and a battery as a power source in order to cope with abrupt load changes and the like exceeding the power generation capacity of a fuel cell.
In a hybrid fuel cell system, the output voltage of the fuel cell and the output voltage of the battery are controlled by a DC/DC converter. As a DC/DC converter which performs such control, a type which performs the conversion of voltage by causing PWM operation of a switching element, such as a power transistor, an IGBT and an FET, is in widespread use. With power savings, downsizing and higher performance advancing in electronic equipment, it is desired that a DC/DC converter provide lower losses, higher efficiency and lower noises and in particular, the reduction of switching losses and switching surges that accompany PWM operations is desired.
There is a soft switching technique as one of the techniques for reducing such switching losses and switching surges. Soft switching is a switching method for realizing ZVS (Zero Voltage Switching) or ZCS (Zero Current Switching), and switching losses of a power semiconductor device and stresses given to the power semiconductor device are low. In contrast to this, the switching method which involves turning on and off voltage and current by the switching function of a power semiconductor device is called hard switching. In the following descriptions, the method by which both or one of ZVS and ZCS is realized is called soft switching, and other methods are called hard switching.
Soft switching is realized by a system in which an auxiliary circuit provided with, for example, an auxiliary switch for reducing switching losses is added to a general boost type DC/DC converter provided with a main coil, a main switch and a diode (what is called a soft switching converter)(refer to Patent Literature 1, for example).